AQS源码-ReentrantLock的源码

//默认是非公平锁 true是公平锁，false是非公平锁
ReentrantLock reentrantLock = new ReentrantLock();

state为什么是voliate修饰
volatile + CAS实现非阻塞式线程同步：通过Syncronized的方式当然可以实现线程同步，但是消耗太大；AQS框架使用volatile + CAS实现非阻塞式线程同步，线程同步需要保证3个方面：可见性、有序性、原子性，volatile保证了变量的可见性和有序性，用CAS *** 作保证变量的原子性。

一、非公平锁（NonfairSync）
1、lock()

final void lock() {
			//如果当前没没有线程拿到锁，给当前线程加锁
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            //如果已经有线程获得锁
            else
                acquire(1);
        }

2、acquire(1)

public final void acquire(int arg) {
		//尝试获取锁   添加等待线程   
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

3、tryAcquire(arg)
作用：获取锁，同时包含记录重入锁

final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            //当前线程拿到锁了
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            //重入锁：已经有线程拿到锁，并且是当前线程，state++
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

4、addWaiter(Node mode)
双向链表，添加等待的线程，head<=>node<=>tail

private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        //从第二个开始的线程，移动tail指向当前线程
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        //第一个线程，head(new Node())<=>tail(node第一个线程)
        enq(node);
        return node;
    }

5、enq(node)
第一个线程添加到双向链表

 private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

6、acquireQueued(final Node node, int arg)

final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                //检查双向链表的第一个node是否获得锁
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //waitStatus，等待状态默认是0，把双向链表的当前节点之前的节点状态更新成-1，等待解除阻塞
                if (shouldParkAfterFailedAcquire(p, node) &&
                	//阻塞当前线程
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

7、unlock

		 * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
            	//释放后继者
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

 private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

二、公平锁
公平锁和非公平锁的区别就是，非公平锁首先还尝试获取锁，公平锁直接阻塞，以下代码：

final void lock() {
            acquire(1);
        }